Sol-gel chemistry has emerged as an attractive—simple, rapid and inexpensive—alternative synthesis route to physical and chemical deposition methods. The relatively mild synthesis conditions and flexibility of sol-gel chemistry have opened the door to a large variety of compounds that can be processed into thin films, fibers, and monoliths. In addition, when combined with molecular and/or colloidal assembly methods, it is possible to create composites with internal three-dimensional meso-to-macroscale periodic features and other hierarchical structures. This large diversity of accessible compositions, morphologies and internal structures makes sol-gel chemistry-derived materials attractive candidates for numerous applications ranging from catalysis, separation and sorption to uses in electronics and photonics.
A remaining challenge for widespread commercial applications is to produce sol-gel-derived materials with structural uniformity over large sample areas (e.g., >cm2) and controlled batch-to-batch reproducibility. This is particularly true for thin-film based materials and applications. Here, the level of uniformity and control over thin-film parameters has been set very high by physical and chemical deposition methods, such as atomic layer deposition, chemical vapor deposition, molecular beam epitaxy and various sputtering techniques. Therefore, for sol-gel processing to be considered a viable fabrication alternative for commercial applications, similar standards have to be achieved. The two foremost challenges in sol-gel fabrication of thin films are the avoidance of crack formation and the achievement of a uniform and reproducible film thickness over large areas. Both of these problems are directly related to solvent evaporation during deposition of thin films (dip or spin coating, casting) and their densification in subsequent heat treatments (calcination, annealing). Conducting the thin film deposition process under controlled environmental conditions (for example, a dedicated clean room with controlled humidity, temperature, substrate pre-treatment, absence of dust particles in air) can help to significantly increase the film quality. However, despite these advances, sol-gel derived thin films are still prone to irregularities in film thickness (edge effects), and shrinkage and crack-formation during heat treatment.
Among the large family of interesting thin film compounds the wide bandgap semiconductor titania has recently gained tremendous attention due to its outstanding chemical and physical properties. While high chemical stability and catalytic activity make titania a prime candidate for photoanodes in solar cells and water-splitting, its high refractive index combined with optical transparency in the visible range of the electromagnetic spectrum are ideal for producing reflective coatings and other optical components. Furthermore, by depositing titania thin films alternatingly in stack-form with a lower refractive index compound such as silica, interesting one-dimensional (1D) photonic bandgap materials can be fabricated, including reflectors, filters, and microcavities. Such layering of thin films of different compounds with different thermal expansion coefficients, however, poses a big challenge for sol-gel fabrication. During film processing and thermal treatments, different shrinkage properties within alternating layers and at interfaces induces stresses and can lead to severe crack formation, delamination, and variation in the film thicknesses.
A strategy for minimizing crack formation in titania/silica thin-film stacks is the firing process. This is an additional short heat-treatment step at high temperatures (900-1000° C.). The purpose of this firing process is to take advantage of the opposing thin film stresses for silica and titania at 900° C. While, studies by Rabaste et al. and Kozuka et al. revealed an induced tensile stress for both silica and titania thin films up to 800° C., increasing the temperature to 900° C. resulted in a compressive stress for silica. It is argued that these opposing stresses result in an overall relaxed multi-stack and reduces the formation of cracks. Contrasting this high-temperature approach, Keszler et al. reported a low-temperature (5° C.) solution-processing method followed by an annealing step at modest temperatures (300° C.). This method allows for the deposition of dense titania thin films. However, in this method the deposition thickness is limited to ˜18 nm for each deposition cycle. To create alternating stack structures with each layer having a thickness of several tens to hundreds of nanometer thickness (as required for application as photonic band gap materials operating in the visible or near infrared range), this method would require deposition of multiple consecutive films of the same compound.